NJC H2 Chem 2012 Prelim P2 Soln

NJC H2 Chemistry Prelim Solutions Solutions to Paper 2 1 (a) (i) LiOH NaOH KOH RbOH CsOH · [1/2] · Size of cations increases down Group II, heat of hydration of cation less exothermic [1/2] Most students can score these two marks. The students who drew an increasing line above the x-axis were awarded one upon two. Common mistakes · It is clearly stated in the question that all of the heats of solution are exothermic. Many students did not seem to notice that, judging from their answers. Their lines were above the x-axis. · It is ∑nΔH(hyd), not just nΔH(hyd)! · A lot of students, while getting the full credit for the graph, failed to include the equation in their explanation. (b) The group I hydroxides are all hygroscopic. [1/2] Weigh the solid out promptly so as to minimise its exposure to air. [1/2] (c) Wear gloves. [1/2] The chemicals are corrosive. [1/2] Common mistakes · Toxicity is not the same as corrosiveness! · Grammar! A pair of gloves, not just a glove! If you were to use just one glove, won’t your other hand get corroded? A pair of google s, not just a goggle! If you were to use just one goggle, how are you going to wear it? Hold it over your eye? (d) Procedure Major points [1/2 each, maximum 2] · Measure the initial temperature of the temperature of the water. · Measure the highest/maximum temperature reached. (No marks if temperature is lowest/minimum; ECF according to answer to (a)). · Measuring cylinder to measure out the water. · Weigh mass of the residue and weighing bottle after adding the LiOH into the cup. Minor points [1/2 each, maximum 1] · Weigh out LiOH using a weighing bottle. · Cup to be supported with a beaker. · Repeat the experiment until two values are within 5 % of each other. ‘m’ has to be specified as the mass of the solid. 1

NJC H2 Chemistry Prelim Solutions 2 Preliminary calculations [1] · Justify mass of LiOH weighed out. [NOTE: The capacity of the cup must be stated, otherwise at most half a mark will be awarded.] [Set ] where m is between 50 to 90 % of the stated capacity of cup. Simplifying, If the capacity of the cup is 100 cm3 and filling 90 % of it, then (e) Volumes of the two solutions are the same/same setup/same apparatus [1] (f) (i) · for both experiments. (Check: and ) · no. of moles of H2SO4 half that of CH3COOH. · total volume not more than 90% of capacity of cup and not less than 50% of capacity of cup. · acids are the limiting reagents i.e. C1V1 < 60 and C2V2 < 30. 2 (a)(i) n(O2) = n(S2O32-) = ( ) = [2] [O2] = ( ) = moldm-3 [1/2] [O2] = ( ) = 7.44 mgdm-3 [1/2] (a)(ii) The carps will be able to survive. (Justification).

NJC H2 Chemistry Prelim Solutions 3 (b)(i) (ii) 2H+ + 3SO3 2- + 2ReO4 - H2O + 3SO4 2- + 2ReO2 [1] Ecell0 = (0.3 – 0.17) V = 0.13 V > 0 [1] Dark solid forms. [1] 3(a) Copper, brass and bronze all have metallic bonds. Copper is a pure metal made